1. Field
This disclosure relates to super-resolution optical microscopy, especially beneficial for identification and alignment of structures fabricated in semiconductor materials.
2. Related Art
Limited optical resolution and poor CAD overlay accuracy is presently limiting post fabrication characterization, design debug, and failure analysis, as well as monitoring of yield issues before completing fabrication.
During design and verification stage of semiconductor devices, it is important to test for various defects or problematic areas of the chip design that may lead to defects. However, in order to do that, many times the device is probed or observed using an optical systems and/or microscopes. Also, in order to identify the structures being imaged, the image is aligned to a CAD design of the circuit. Typically, this step includes aligning corresponding features of two different images of the IC under test. The first image can be an acquired image of the actual circuit. The second image can be derived from a computer-aided design (“CAD”) image that lays out the complicated map of circuit elements. In general, a CAD image is ideal representation of the IC and typically is generated using a CAD system. Under such conditions, it is important to be able to optically resolve various features in the image of the actual IC, such that the features can be aligned to the CAD image.
FIGS. 1A and 1B illustrate a conventional diffraction-limited reflection-based imaging system for ICs. The example illustrated in FIG. 1A utilizes a computer system 100, which controls and receives signals from an illumination source, shown as a continuous-wave laser source (e.g., 1064 nm). The light from the illumination source 105 passes through beam shaping optics 110 and scanning elements, e.g., Y-galvanometer controlled mirror 120 and X-galvanometer controlled mirror 125, together forming a confocal laser scanning microscope (LSM). The Y-mirror and X-mirror scan the beam in a slow scan direction and a fast scan direction, respectively, as illustrated in the callout. The bean then passes through a high numerical aperture objective lens 130 and a solid immersion lens 135, to be scanned on an area of interest in the specimen, e.g., an IC circuit generally referred to as device under testing (DUT) 140.
FIG. 1B illustrates another example which uses a light source 160, which may be, for example, LED, super luminescent diode (SLED or SLD), laser, etc. Its light beam is passed through aperture 165 and beam shaping optics 110. Beam splitter 150 directs the beam towards DUT 140, via objective lens 130 and SIL 135. Light reflected from the DUT 140 is collected by the SIL 135 and passes objective 130 and beam splitter 130, to be collected by camera 155. The signal of the camera is sent to the computer 100.
The above examples enable optimized navigation, visual characterization and CAD overlay of structures of interest located generally between 10-100 microns, and sometimes even up to 780 microns, below the IC's silicon substrate, depending on substrate doping and sample preparation. Although these approaches operate with a high level of performance control, they are ultimately limited as they cannot address the demands faced by the latest fabrication process nodes. More specifically, they lack the optical resolution required to achieve both accurate structural definition and recognition in order to fulfill the duties outlined above. This therefore requires a novel imaging system in order to match these developments.
For further information the reader is directed to: S. B. Ippolito, B. B. Goldberg, and M. S. Unlu, “High spatial resolution subsurface microscopy”, Applied Physics Letters 78, 4071-4073 (2001); S. B. Ippolito, B. B. Goldberg, and M. S. Unlu, “Theoretical analysis of numerical aperture increasing lens microscopy”, Journal of Applied Physics 97, 053105 (2005); K. A. Serrels, E. Ramsay, P. A. Dalgarno, B. D. Gerardot, J. A. O'Connor, R. H. Hadfield, R. J. Warburton, and D. T. Reid, “Solid immersion lens applications for nanophotonic devices”, Journal of Nanophotonics 2, 021854 (2008); K. A. Serrels, E. Ramsay, R. J. Warburton, and D. T. Reid, “Nanoscale optical microscopy in the vectorial focusing regime”, Nature Photonics 2, 311-314 (2008); K. A. Serrels, E. Ramsay, D. T. Reid, “70 nm resolution in subsurface optical imaging of silicon integrated-circuits using pupil-function engineering”, Applied Physics Letters 94, 073113 (2009); D. A. Pucknell and K. Eshraghian, Basic VLSI Design, 3rd edition, Prentice Hall (1994); and U.S. Pat. Nos. 7,659,981; 7,616,312; 6,848,087; and 6,252,222.